The use of renewable energy sources is becoming increasingly necessary to reduce emissions of carbon based fuels and provide alternatives to petroleum based energy and processing hydrocarbon-based feedstocks. One of the process alternatives being explored is the pyrolysis of biomass. Biomass is any carbon containing material derived from living or formerly living organisms, such as wood, wood waste, crops, crop waste, waste, and animal waste.
Biomass can be processed using several techniques. One increasingly popular technique for processing wood-based feedstocks. Pyrolysis, which is the thermal decomposition of a substance into its elemental components and/or smaller molecules, is used in various methods developed for producing hydrocarbons, including but not limited to hydrocarbon fuels, from biomass. Pyrolysis requires moderate to high temperatures, generally greater than about 325° C., such that the feed material is sufficiently decomposed to produce products which may be used as hydrocarbon building blocks.
Generally the pyrolysis of biomass produces four primary products, namely water, “bio-oil,” also known as “pyrolysis oil,” char, and various gases (H2, CO, CO2, CH4, and other light hydrocarbons) that do not condense, except under extreme conditions.
Fast pyrolysis is one method for the conversion of biomass to bio-oil with high yields. Fast pyrolysis is the rapid thermal decomposition of organic compounds in the absence of atmospheric or added oxygen to produce liquids, char, and gas. Generally, fast pyrolysis uses dry (<10% moisture) feedstock of biomass comminuted into small particles (<about 3 mm), moderate temperatures (325-750° C.), and short residence times (0.5-2 seconds). This pyrolysis reaction may be followed by rapid quenching to avoid further decomposition of the pyrolysis products and secondary reactions amongst the pyrolysis products.
Fast pyrolysis affords operation at atmospheric pressure, moderately high temperatures, and with low or no water usage. Bio-oil yields typically range from 50-75% the mass of input biomass and are heavily feedstock dependent. Generally, known methods of bio-oil production result in bio-oil with high oxygen (>50 wt %) and water content (>30%); such oxygen and water content may result in storage instability and phase-separation issues.
For example, the pyrolysis of a wood based biomass will produce a mixture of organic compounds such as lignin fragments, aldehydes, carboxylic acids, phenols, furfurals, alcohols, and ketones, as well as water. Unfortunately, compounds such as the aldehydes and acids may react with other components of the bio-oil, creating instability, corrosiveness, and poor combustion characteristics.
Bio-oil typically requires additional upgrading in the presence of a catalyst and/or hydrogen to be used in transportation fuel applications. These upgrading steps can be integrated into the existing pyrolysis unit or used in post-treatment schemes. Shabtai, et al., U.S. Pat. No. 5,959,167, use a catalytic reaction process to produce a reformulated hydrocarbon gasoline product. Marker and Petri, U.S. Pat. No. 7,578,927, convert pyrolytic lignin material into naphtha and diesel boiling range components, having low acidity and ultra-low sulfur content. Zmierczak and Miller, US2008050792, use a base catalyzed depolymerization (BCD) reaction to produce a partially depolymerized lignin for further processing to fuel range products. Baldiraghi and associates, WO2008113492, describes a process using hydrodeoxygenation followed by hydroisomerization on an acidic SiO2/Al2O3 catalyst. Bzdek and Pellegrino, US2008092435, provide biodiesel fuels prepared by removing deleterious chemical species from the fuel to insure the filterability of the fuel, both neat and in various biodiesel fuel blends. Kleinert, et al. (2009), convert lignin residues from lignocellulosic ethanol production into organic liquids with a high hydrogen to carbon (H/C) and a very low oxygen to carbon (O/C) ratio.
Upgrading pyrolysis oils is difficult due to acidity of the pyrolysis oil, contamination with other compounds, and the tendency to form coke by-products. Damaging costly cracking catalysts is expensive and removes any profit margins from processing biomass and pyrolysis oil to high value hydrocarbons. Therefore, it would be desirable to have a method of cracking biomass, decreasing acidity by removing the organic acids and upgrading pyrolysis oil into useful products in a cost effective manner.